In addition to the mainstream Marvel Universe (designated Earth-616) version of the character, Black Widow has numerous alternative versions throughout the Multiverse.
1602
In Marvel 1602, a world where superheroes have started to appear several hundred years early, Natasha is a freelance spy and "the most dangerous woman in Europe." Initially allied with Matthew Murdoch (Daredevil's 1602 counterpart), she later betrays him to Count Otto Von Doom, whom she shares a romantic relationship with. It is Natasha who later comes to Doom's rescue when he is horribly burned by Thor's lightning & the accompanying discharge of Blake's golden sphere.
Natasha is still working with Count von Doom during Marvel 1602: Fantastick Four, when she is captain of his flying ship. However, when she questions his plan to take the ship to the edge of the world, he pushes her over the side and appoints the Wizard captain.
In the Age of Ultron story, Black Widow is shown after Ultron's attack with disfiguring scars on the right side of her face from an unknown source, disarming a desperate and panicked man who attempts to threaten her for resources before meeting up with Moon Knight in one of Nick Fury's old bases.
The Avengers: United They Stand
Natasha Romanoff/Black Widow appeared in the comic-book series based on the animated television series The Avengers: United They Stand.
Earth X
Natalia Romanova appeared in the Earth X miniseries much the same as her 616 counterpart. She has was given a piece of the Absorbing Man to protect after his destruction which led to the death of the Avengers. Later in the series she is revealed in the Realm of the Dead and is recruited by Mar-Vell into his legion of the undead.
House of M
Natasha is seen as a member of the Soviet Super Soldiers. Natasha later appears a member of Shang-Chi's Red Dragons, and is mentioned as a member of S.H.I.E.L.D.
Marvel Mangaverse
She appears in the New Dawn arc where The Executioner and she are hired by Mordo to kidnap Bruce Banner. They manage to succeed in doing so, with her escaping with Bruce while Tigra is dealing with The Executioner intent on keeping the reward for the job herself. However during her escape, Bruce turns into The Hulk and destroys the submarine they were in, presumably killing her.
Marvel Zombies
Black Widow is among the Avengers who are infected by the Sentry in Marvel Zombies vs. the Army of Darkness. She is seen consuming a Yorkshire Terrier puppy. Later, in the original Marvel Zombies series, she is among the zombies who attack the Silver Surfer.
Ultimate Marvel
Under the Ultimate Marvel imprint Natasha Romanova is a member of the Ultimates, this universe's analogue of the Avengers. She debuted in Ultimate Marvel Team-Up No. 14 (June 2002) in a story written by Brian Michael Bendis and drawn by Terry Moore, before becoming one of the major characters in writer Mark Millar and penciller Bryan Hitch's The Ultimates, debuting there in No. 7 (Sept. 2002).
Romanova is a former KGB spy and assassin, code-named the Black Widow, known for killing her ex-husbands, notably Alexi Shostakov. She was originally part of the Ultimates' covert operations ("black ops") team, but with the emergence of the Chitauri threat was subsequently moved to public status after a publicly acceptable background was written for her. She has genetic or cybernetic enhancements making her far better in combat than the average human. After accepting a marriage proposal from Tony Stark, she receives a black suit of Iron Man armor as an engagement present, along with a set of nanites to control the armor.
Romanova is later revealed as a traitor, responsible for killing Hawkeye's wife and children, revealing Bruce Banner's connection to the Hulk to the public, and collaborating with the Liberators in their invasion of the United States. After she murders Edwin Jarvis and holds Stark hostage in an attempt to extort his fortune, Stark activates the nanites in her bloodstream, freezing her body in place, before knocking her unconscious with a wine bottle. She appears later in an emergency clinic, having cut open her wrists to bleed out the disabling nanites. Hawkeye kills her in revenge for her part in the murder of his family.
By Ultimates 3 #1 a sex tape between Natasha and Stark has been leaked to the public. According to Janet Pym, the tape is of unknown origin and seems to have been professionally done with "close ups". Stark is shown to be in a state of depression over Natasha's betrayal and subsequent death. It is later revealed that one of the Ultrons calling himself Yellowjacket had leaked the tape. During the Brotherhood's attack on the Stark Mansion, Mystique shapeshifts into Natasha to get close to Tony in an attempt to kill him but is knocked unconscious by Wasp.